1. Field of the Invention
This invention relates to graphite-hydrogensulfate and, more particularly, refers to a new and improved process for the treatment of graphite to produce graphite-hydrogensulfate.
2. Description of the Prior Art
It was known in the production of expanded graphite, to wet flaky graphite particles with an oxidizing acid or another oxidation agent or to treat them in an acid or mixture of acids, and to dissociate the graphite salt which is formed in the reaction by rapid heating. During the heating, the volume of the graphite particles increases considerably, for example, more than hundred times, because of the gas that is suddenly generated. In graphite of a high degree of structural order or crystallinity, the particles spread apart, similar to the pages of an opening book, in a direction perpendicular to the laminar planes of the graphite crystals, in such a manner that the coherence within the particles is never completely lost. The bloated particles which are known under the name of expanded graphite show a characteristic harmonica-like or worm-like structure, and are easily pressed to form shaped parts, whereby the particles, due to their unusual structure, lock in with each other, like teeth, so that the use of an additional binding agent is not required. Expanded graphite in the form of odd particles and the shaped parts produced from the latter are an excellent material for seals of all kinds, among other reasons because of their plasticity, unusual for graphite.
For the production of graphite salt which is the preferred starting material for the making of expanded graphite, numerous compounds or compound mixtures are known as oxidation agents, such as fuming nitric acid, solutions of nitric acid, solutions of perchlorate or chromate in sulfuric acid, potassium-permanganate, lead dioxide and others. The graphite particles are wetted with the oxidation agents or dispersed in them, and the formed graphite salts are washed for the removal of acids and oxidation agents when the reaction is completed. Since it is not possible even by the most thorough washing, to remove the oxidation agents completely from the graphite salt, the remainder of these agents unfavorably influences the corrosion behavior of the expanded graphite product made from the salt. Oxidation agents which contain metal-ions cannot be used because of their corrosive activity. Other oxidation agents, such as fuming nitric acid or ammonium persulfate can only be safely handled with considerable difficulty, and further require an unduly long time for treatment because of their slow reaction speed. The commonly used agent presently used is a mixture consisting of concentrated sulfuric acid and fuming nitric acid, in a ratio of 10:1 to 2:1. This mixture can be comparatively safely handled and thoroughly oxidizes the graphite in the reaction, producing a reaction product designated graphite-hydrogensulfate. Unfortunately, this mixture has the decided disadvantage of forming nitrogen oxides, particularly nitrogen dioxide, during the reaction with the graphite. Part of these nitrogen oxides escape directly into the atmosphere, and part of the nitrogen oxides is dissolved in the sulfuric acid and then escapes as nitrous vitriol gas during the separating-purification operations which are required for the separation of the formed graphite salt.
The continuous generation of nitrogen oxides, from the first contact of the acid mixture to the final treatment of the graphite salt makes costly preventive measures for the protection of the environment mandatory, and even such measures do not completely avoid pollution of the environment. Also disadvantageous is the high water content of the acid mixture after the reaction because, in addition to the reaction-water which cannot be avoided, water is brought in with the nitric acid. If it is attempted to increase the concentration of the sulfuric acid by the addition of oleum, nitrous vitriol gases are developed in large amounts in sudden bursts. These gases can only be partly extracted with considerable effort by filter installations and washing. Other methods for concentration of acid remaining after treatment of graphite, are complicated by the fact that, after the reaction, the acid mixture contains graphite or graphite oxides in colloidal form. The expenditure in technique, time and money to raise the concentration is so great, particularly in smaller installations for the manufacture of expanded graphite, that as a rule, reclaiming of the acid is not attempted and the acid is neutralized and then drained to the sewage system.